Ocean Acidification: Solutions, Impact and Causes

Introduction

Ocean acidification, an ongoing decrease in the pH of the Earth’s oceans, is mainly caused by the increasing amount of carbon dioxide and the rising Arctic Ocean temperature. Statistically, during the last two hundred years, the ocean pH has dropped by thirty percent globally (Orr et al., 2005), meaning that this change is large enough that ocean acidification already has the potential to affect some of the oceans and the biologically important residents. This calls for some drastic ocean acidification solutions.

Understanding Ocean Acidification

The oceans absorb most carbon dioxide from the atmosphere, which plays a critical role in regulating climate, however, the unprecedented amount of carbon dioxide being created nowadays has surpassed what the ocean can normally absorb, changing ocean chemistry and making them more acidic (Orr et al., 2005). The rise in atmospheric carbon dioxide mainly results from increased fossil fuel use and deforestation.

Causes and Impacts of Ocean Acidification

Humans burn large amounts of fossil fuel for energy uses including gasoline for cars, heating oil, and natural gas used to generate electricity. 37% of global emissions are from fossil fuels traded internationally (Davis et al., 2011). Besides energy use, significant fractions of fossil fuels are used for non-energy applications. When fossil fuels are used for non-energy purposes, there are several ways to show how this can ultimately lead to carbon dioxide emissions. Chemical commodities such as solvents may lead to carbon dioxide emissions after use as a consequence of oxidation in the atmosphere, for example, after the application of a solvent-based paint with a brush in open space. Another pathway of carbon dioxide originating from non-energy use is certain industrial processes. If part of the feedstock is oxidized in the chemical conversion like in the case of hydrogen production, then this is typically considered as an inherent feature of the chemical process and not as fuel combustion. In this case, the resulting carbon dioxide is referred to as industrial process emissions (Freed et al., 2005). Deforestation is also mainly caused by human elements. Human populations play a direct role in deforestation by clearing land for gardens, cutting trees to obtain timber, firewood, and etc. Also, in many forested areas, native species of trees have been replaced by economically valuable introduced species. And human colonization introduces fire as a powerful force in the island's deforestation. It is particularly significant in the deforestation of dry islands because traditional agricultural systems often rely on fire for clearing fields, and on dry islands where there is a high risk of fires spreading and burning out of control (Van der Werf et al., 2010).

The release of methane from melting hydrates in shallow regions of the Arctic Ocean could exacerbate ocean acidification in the water column. Hydrate destabilization can occur in the Arctic in response to global warming and that the potential methane release is substantial but limited in the next 100 years (Biastoch et al., 2011). Vast amounts of methane hydrates are potentially stored in sediments along the continental margins, owing to their stable low-temperature — high-pressure conditions. Global warming could destabilize these hydrates and cause a release of methane (CH4) into the water column and possibly the atmosphere. The resulting warming is spatially inhomogeneous, with the strongest impact on shallow regions affected by Atlantic inflow. Within the next 100 years, the warming affects 25% of shallow and mid‐depth regions containing methane hydrates. The release of methane from melting hydrates in these areas could enhance ocean acidification and oxygen depletion in the water column (Biastoch et al., 2011).

Effects of Ocean Acidification

Changes in marine ecosystems and economic devastation are two major effects with ongoing ocean acidification. Coral reefs, an ecosystem recognized as vulnerable to ocean acidification, have started to show signs of decline that may be due to ocean acidification. Some of the largest reef-building corals on the Great Barrier Reef are showing more than 14 percent reductions in skeletal growth since 1990 (De'ath et al., 2009). Sea turtles are some of the most endangered marine animals and are often found resting and feeding within coral reefs. As ocean acidification worsens the abundance of reef species will likely diminish, which could result in turtle feeding behaviors and could cause them to turn to less nutritious food sources or even go hungry (Bonin et al., 2006). Besides related marine organisms, healthy reefs provide goods and services to society, including fisheries, coastal protection, tourism, education, and aesthetic values. In Hawaii alone, coral reefs annually generate 364 million dollars through tourism. If reefs collapse because of rising acidity, global warming, and other threats, coastal communities will bear the brunt of these losses (Sukhdev et al., 2010). Serious health consequences could result in the estimated 30 million people who rely almost solely on coral reef ecosystems for protein and protection. The potential losses from a decline in coral reefs will be felt from the smallest coastal subsistence communities all the way up through the global economy (Wilkinson & C, 2008). Furthermore, many scientists estimate that the main reef-building organisms, corals and calcifying macroalgae, will calcify 10-50% less relative to pre-industrial rates by the middle of this century. This decreased calcification is likely to affect their ability to function within the ecosystem and will almost certainly affect the workings of the ecosystem itself (Kleypas & Yates, 2009).

However, ocean acidification affects not only the corals but also the reefs they build. The decline in calcium carbonate production, coupled with an increase in calcium carbonate dissolution, will diminish the reefs and the benefits that reefs provide like high structural complexity that supports biodiversity on reefs and breakwater effects that protect shorelines and create quiet habitats for other ecosystems, such as mangroves and seagrass beds (Kleypas & Yates, 2009). By the middle of this century, if carbon dioxide emissions continue unabated, coral reefs could be eroding through natural processes faster than they can grow their skeletons due to the combined pressures of increasing acidity and global warming (Silverman et al., 2009). Reefs may greatly change from the structures that so many species rely upon for habitat, which means that when corals face severe declines and even extinction, the survival of reef dependent species will be in turn threatened. Although they cover just over one percent of the world’s continental shelves, coral reefs serve as important habitat to more than twenty-five percent of all marine fish species (Knowlton et al., 2010). As reef habitat becomes less available, coral reef-dependent fish will decline as a result. Coral bleaching event is an example to explain the relationship between coral reefs and coral reef-dependent fish species. For instance, after one event in Papua New Guinea, 75 percent of the coral reef fish species declined in abundance and several species even wet extinct (Jones et al., 2004). As early as the year 2050, pteropods may be unable to form calcium carbonate shells, which would threaten their ability to survive (Orr et al., 2005). If they cannot adapt to living in more acidic waters, their population will plummet, which could affect the food webs that depend on them (Doney et al., 2009). The North Pacific salmon that depend heavily upon pteropods for food (Aydin et al., 2005). And the North Pacific salmon fisheries provided three billion dollars worth of personal income to fishermen and others in 2007 and supported 35 thousand jobs in just the harvesting and processing of the fish (Orr et al., 2005). Hence, with the decrease of the pteropods, the North Pacific salmon and other commercially important fish species that eat pteropods including mackerel and herring would risk collapse and directly result in the declines of personal or fishery income and job losses. Also, declines in the smallest of species, like pteropods and salmon, could reverberate throughout the oceans, ultimately impacting the largest marine species. For example, the Chukchi and Northern Bering Seas are some of the richest fishing grounds in the world and are home to predators as varied as gray whales, seals, and walruses that all depend on marine calcifiers for food. Resident killer whales in the North Pacific prefer to eat salmon, nearly 96% of some killer whales’ diet is made up of salmon (Fabry et al., 2009). When the base of the food web disappears, the top food web disappears immediately as well. If top predators are unable to supplement their diet with other food sources, food webs may even collapse entirely.

Ocean acidification may also affect shellfish species like sea urchins, damselfish, and brittle stars. Sea urchins on a reef, crucial grazers in any environment, help to protect the reef by eating some of the algae. They reproduce by releasing eggs and sperm directly into the surrounding seawater. However, sperm of some sea urchins swim more slowly under acidified conditions (Reuter et al., 2010), which reduces their chances of finding and fertilizing an egg, forming an embryo and developing into sea urchin larvae (Havenhand et al., 2008). The majority of sea urchin embryos and larvae are eaten by fish and only a few survivors mature into adults as a result. Although sea urchins normally release millions of eggs and sperm into the surrounding water to make up for this low success rate, scientists predicted that the more acidic conditions could reduce the number of sperm certain species release, thereby further decreasing the size of the next generation of sea urchins by the end of this century (Reuter et al., 2010). What’s more, like many other calcifiers, such as corals, pteropods, and oysters, sea urchins are likely to find it more difficult to build their calcium carbonate skeletons in an acidified ocean. Young sea urchins have been observed to grow slower and have thinner, smaller, misshapen protective shells when raised in acidified conditions. Slower growth rates and deformed shells may leave urchins more vulnerable to predators and decrease their ability to survive (Brennand et al., 2010). As a result, slower shell growth is likely to reduce the ability of mollusks to survive, which would have significant impacts on commercial fisheries. If this slowed growth kept continuously, mollusk fisheries would have lost between 75-187 million dollars (Cooley & Doney, 2009). In addition to smell, some reef fish like the damselfish rely on hearing to find their way back to their home reef. They listen to the noises of a reef using otoliths, which are calcium carbonate structures similar to human ear bones. Using their otoliths, fish larvae can separate the low-frequency sounds of breaking waves, currents, and surface winds of the open ocean from the high-frequency sounds of gurgling, cracking and snapping of a coral reef. Damselfish larvae use these distinct noises to navigate back to their home reef and away from the open ocean (Gagliano et al., 2008). However, carbon dioxide concentrations expected around the end of this century have been observed to alter the normal development of otoliths in the larvae of an open ocean fish, white sea bass (Asch, R. 2009). The enhanced otolith growth could make it difficult for the fish to locate appropriate reef habitat and result in population declines. Larger than normal otoliths in damselfish have been shown to decrease their ability to recognize sounds and return to a coral reef (Gagliano et al., 2008). Brittle stars also play a crucial role in the environment as burrowers and as a food source for larger predators like flatfish. The spindly arms of a brittle star break off when the animal senses danger and, under normal conditions, can quickly regenerate. Although brittle stars can still regenerate their arms under acidic conditions but do so with less muscle mass than usual (Gooding et al., 2009). The brittle stars not only created insufficient amounts of muscle for their new arms to function correctly, but they also devoured muscle from their already existing arms to provide energy for the now much harder process of building calcium carbonate. Weakened arms could decrease the ability of brittle stars to survive in a more acidic ocean. Increased acidity is also likely to threaten brittle star larvae (Dupont et al., 2010). It appears that brittle stars appear to be very vulnerable to increasing ocean acidity both as adults and larvae, which would result in serious population declines in the future.

In addition to shellfish species, animals without shells or skeletons will also be affected by ocean acidification like clownfish and cardinalfish. Larvae of fish that live on coral reefs hatch on the reef and migrate to the open ocean where they spend the next two to three weeks adrift. When the larvae are ready to return to their home reefs, they use their sense of smell and sound to guide them back (Munday et al., 2009). Unfortunately, under the more acidic conditions, larvae may not be able to discern between the smells of a suitable home and a hostile environment, which could finally result in their death (Dixson et al., 2010). Additionally, clownfish also use their sense of smell to avoid predators. While in higher carbon dioxide conditions expected around the end of this century, this smell-related predator defense system is disrupted and most returning clownfish larvae are no longer able to discern between predator and non-predator cues (Munday et al., 2010). And increased levels of carbon dioxide in seawater may decrease the ability of some fish to breathe like cardinalfish. Cardinalfishes have been found to be particularly vulnerable to increasingly acidic conditions. The ability to take up oxygen decreased by as much as 47 percent in one species of cardinalfish when exposed to carbon dioxide levels similar to those expected by the end of this century. The reduced ability to breathe will more likely to impact cardinalfish including decreased ability to feed, grow and reproduce, which may result in adverse consequences for the sustainability of cardinalfish populations. As the acidity of the ocean increases, they are simultaneously getting warmer due to climate change. Increased temperature combined with the acidity levels expected by the end of this century proved lethal for one species of cardinalfish tested in the laboratory. These results are particularly concerning, especially whether they are found to apply to other species since they show that while individuals might be able to survive one threat, they are less able to endure the simultaneous threats of increasing temperature and ocean acidification (Munday et al., 2009).

Clean-Energy Solutions to Ocean Acidification

Preserving natural resilience and reducing carbon dioxide emissions are potential preventions that stop ocean acidification becoming worse to severely affect the marine ecosystems and economic devastation further. To preserve natural resilience, we need to curtail human-caused threats especially the overfishing to maintain the natural resilience of marine ecosystems. Overfishing has profoundly affected the world’s oceans both directly and indirectly. For example, fisheries scientists recently estimated that over the past 50 years the global biomass of large predatory fish – such as tuna and swordfish – has declined by 90 percent and that the diversity of these fish has declined 10-50 percent (Myers & Worm 2003; Worm et al. 2005). The decline of fish populations is often particularly hard on poor coastal communities – in both the global North and South – where many people depend on fishing (and fishing-related industries, such as boat building and fish processing) for food and employment. The crisis of overfishing, then, has both environmental and socio-economic dimensions because overfishing is a problem for fish, their ecosystems, and people that depend on them (Mansfield, 2010). Home energy and personal transport are the top two contributors to the average American’s carbon dioxide emissions into the environment, accounting for over 50% of their total carbon footprint. To date, the primary methods applied to improve energy efficiency and/or reducing energy usage have been technological and economic (Armel, 2008). For example, the production of hybrid or hydrogen vehicles has been emphasized as a major solution to CO2 reduction and oil dependence. However, there is growing evidence that a human-centered, behavioral approach should also be pursued to educate, inform, and motivate energy-efficient human behaviors. In-home feedback technology has been shown to reduce energy use by 10-15% on average, with significant decreases linked to more frequent feedback and higher data granularity (e.g., specific energy usage data on appliances). As the cost of home energy-sensing decreases, we will see a huge upsurge in the amount of data available to be visualized and fed back to the consumer about their energy usage. The ways in which to most effectively build interfaces around this data to reduce consumption is an open research question and one that involves psychology and HCI (Froehlich, 2009). Also, reducing CO2 emissions from transportation by diverting traffic demand to less polluting modes has been one of the main priorities of the European policy of the last decades and hence a good number of effective measures have been planned and implemented throughout the Continent (Nocera & Cavallaro, 2011).

Conclusion

In conclusion, we are forcing marine species to live in unusual conditions unlike any that has existed for many millions of years. For some species, the changes we are forcing on them will be so vast that they are likely to be pushed into extinction. Although there definitely will be ecological winners and losers, overall, marine ecosystems will change for the worse. They will become less vibrant and diverse, which results in that many goods and services they provide will dwindle, forcing millions of marine organisms or even people to find new food sources, new homes, and new sources of income. Some of the most vulnerable communities will not have alternatives available to make up for the loss of marine goods and services. Adapting to these losses will take huge resources from the global community and in some cases, adaptation will not be possible. Undesirable species are likely to be among the winners as declines in their direct competitors and predators will allow them to flourish. Some species will even increase their growth rates and abundance because of increasing carbon dioxide (Connell et al., 2010). All the outcomes will be brought by ocean acidification reflecting an overall decline in biodiversity and signify an ocean out of balance. The rising acidity will have widespread impacts on many types of marine life. Non-calcifiers have also exhibited responses that will likely decrease their ability to survive in an acidified future. Therefore, ocean acidification is a global problem in marine ecosystems and the economy of fisheries. It’s necessary and urgent to prevent ocean acidification from becoming worse by preserving natural resilience, reducing carbon dioxide emissions, transiting to cleaner, renewable sources of energy, and preventing the need for large-scale adaptation.  

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